1. Field of the Invention
This invention relates to dry separators of the roll type capable of the separation of feebly magnetic granular materials, which materials also have small differences in their dielectric susceptibility and which may be either paramagnetic or diamagnetic in nature, for the purpose of isolating specific material from an heterogeneous batch.
2. The Prior Art
In the prior art pertaining to magnetic separation, a variety of separators, including roll separators, have been described for the foregoing purpose. It has been demonstrated that for the best separations of similar materials, a high background magnetic field and high field gradients are required. Prior art apparatus having such features has been designed for both dry and wet media.
Two basic types of roll separators represent the prior art:
a. The induced roll separator utilizing a magnetic field developed by an electric current and PA0 b. The Perm Roll separator in which the field is provided by strong permanent magnets.
In both systems the rolls are constructed in such a manner that high magnetic gradients are developed on their respective surfaces. In the case of induced roll separators this is often accomplished in the prior art by in-laying of nonmagnetic material (such as copper wire) into the ferromagnetic surface of the rolls.
In the case of the perm roll separator, the actual roll is formed of a plurality of permanently magnetized annular discs which are assembled on a shaft through their central holes. For the maximum performance of the rolls, the individual magnetic discs are arranged so that only identical magnetic poles such as north and south are facing each other across a "gap" provided by a magnetically soft material comprising a higher magnetic permeability.
Furthermore, in order to keep magnetic materials from building up on a permanent magnetic roll, the state of the art separators comprise a revolving belt wrapped around that roll and an idler roll, which is always detrimental to the performance of the rolls, even with the least interfering belts.
The prior art machines of the above types all comprise feeder devices for the granular materials that are designed to provide variable, but uniform (constant weight per inch of roll) loading of the rolls. I have discovered that this principle is primarily responsible for the necessity for multiple passes of the material over the rolls, because the rolls are often over or underfed, causing particle masking or undesirable particle trajectories to develop and most importantly, a large portion of the feed material is introduced to those sections of the rolls having both the lower field and gradient magnetic product.
The prior art separators of the above types also incorporate a splitter system, most commonly a wedge, which can never be properly placed for all fractions contained in the feed, even though it is known that the roll will act as a particle size classifier in addition to its magnetic effects, resulting often in waste of good material owing solely to its size, rather than its magnetic properties.
Yet another disadvantage of the prior art equipment derives from the use of vibratory feeders for the initial feeding of the first roll in multi-stage roll separators. In particular the feeding of dielectric minerals having high resistivities leads to the build-up of electric charges on the particles prior to their entering the high field/high gradient areas of the magnets, which charges compete and interfere with the magnetic forces to be developed, resulting in indiscriminate separation conditions.
In the prior art of electrostatic or electrodynamic separation, a variety of separation equipment including roll separators and free-fall separators, have been described for the separation of granular materials which have small differences in their dielectric properties.
Compared to magnetic separation techniques, contamination-free electrostatic separation is significantly more complex in nature and if not impossible to realize. In the prior art electrostatic roll separator, vibratory fed and thereby charged particulate matter is deposited onto a rotating grounded metal drum. Non-conductors will adhere to the rotating drum due to image charges developed on the drum while conductors lose their charges rapidly and are spun off by centrifugal forces. Particles being more conductive than others will also be deflected off the rotating drum when entering the electric field of a suitably spaced away electrode, generally being negatively charged, while the non-conductors continue to cling to the drum and must be brushed off. In the case of the non-conductors being the product, such as in the instance of high purity quartz, contamination is inevitable. The same holds true for yet another variation of the prior art separator, the electrodynamic type roll, where the electric charges are provided by a corona field, with all other apparatus features remaining the same.
Yet another type of prior art electrostatic separator, the free-fall type, is a bulky apparatus comprising dozens of feet in height. Extremely high operating voltages (up to 250 Kilovolts) are used for the separation of granular materials with such conventional bulky splitter systems to produce the required deflections of several inches, sometimes feet. Repeated heating of the feed material becomes then mandatory because of the multiple pass requirement for better separation. High temperature is unsuitable for magnetic separation due to the loss of magnetic properties of both the feed and the magnets. Hence magnetic and electrostatic separations have not been combined in prior art apparatus or into a single step in conventional methods.